1. Field of the Invention
This application relates to poly(ortho esters). In particular, this invention relates to bioerodible poly(ortho esters) from dioxane-based di(ketene acetals); and to block copolymers containing them.
2. Description of the Related Art
Interest in synthetic biodegradable polymers for the systemic delivery of therapeutic agents began in the early 1970's with the work of Yolles et al, Polymer News 1:9–15 (1970) using poly(lactic acid). Since that time, numerous other polymers have been prepared and investigated as bioerodible matrices for the controlled release of therapeutic agents.
U.S. Pat. Nos. 4,079,038, 4,093,709, 4,131,648, 4,138,344 and 4,180,646 disclose biodegradable or bioerodible poly(ortho esters). These polymers are formed by a reaction between an orthoester (or orthocarbonate) such as 2,2-diethoxytetrahydrofuran and a diol such as 1,4-cyclohexanedimethanol. The reaction requires elevated temperature and reduced pressure and a relatively long reaction time. Drugs or other active agents are retained in the polymer matrix to be released as the polymer biodegrades due to hydrolysis of the labile linkages.
U.S. Pat. No. 4,304,767 discloses polymers prepared by reacting a polyol with a polyfunctional ketene acetal. These polymers represent a significant improvement over those of U.S. Pat. Nos. 4,079,038, 4,093,709, 4,131,648, 4,138,344 and 4,180,646, since synthesis proceeds readily at room temperature and atmospheric pressure, and the resulting polymers have superior properties.
Further polymers are disclosed in U.S. Pat. No. 4,957,998. These polymers contain acetal, carboxy-acetal and carboxy-orthoester linkages, and are prepared by a two-step process beginning with the reaction between a polyfunctional ketene acetal and a compound containing a vinyl ether, followed by reaction with a polyol or polyacid.
Still further polymers of a similar type are disclosed in U.S. Pat. No. 4,946,931. The polymers are formed by a reaction between a compound containing a multiplicity of carboxylate functions and a polyfunctional ketene acetal. The resulting polymers have very rapid erosion times.
Despite the ease with which the orthoester linkage hydrolyses, poly(ortho esters) known in the prior art are extremely stable materials when placed in an aqueous buffer, or when residing in the body. This stability is attributable to the extreme hydrophobicity of the poly(ortho esters) which severely limits the amount of water that can penetrate the polymer. To achieve useful erosion rates, therefore, acidic excipients must be physically incorporated into the polymer. While this allows control over erosion rates, the physically incorporated acidic excipient can diffuse from the polymer matrix at varying rates, leaving a matrix that is completely depleted of excipient while the polymer still has a very long lifetime remaining.
U.S. Pat. Nos. 4,764,364 and 4,855,132 describe bioerodible polymers, in particular poly(ortho esters) containing an amine functionality. The polymers are said to erode more rapidly at lower pH than at higher pH in an acidic aqueous environment.
Micellar System for Tumor Targeting
One of the major problems in treating cancer is the difficulty of achieving a sufficient concentration of an anticancer agent in the tumor. This is due to the toxicity, sometimes extreme, of such agents which severely limits the amounts that can be used. However, a major discovery in cancer chemotherapy has been the so-called EPR (enhanced permeation and retention) effect. The EPR effect is based on the observation that tumor vasculature, being newly formed vasculature, has an incompletely formed epithelium and is much more permeable than established older vasculature which is essentially impermeable to large molecules. Further, lymphatic drainage in tumors is very poor thus facilitating retention of anticancer agents delivered to the tumor.
The EPR effect can be used in cancer targeting by using delivery systems containing anticancer drugs that are too large to permeate normal vasculature, but which are small enough to permeate tumor vasculature, and two approaches have been developed. In one approach, a water-soluble polymer is used that contains an anticancer drug chemically bound to the polymer via a hydrolytically labile linkage. Such drug-polymer constructs are injected intravenously and accumulate in the tumors, where they are internalized by the cells via endocytosis and released in the lysosomal compartment of the cell via enzymatic cleavage of the labile bond attaching the drug to the polymer. Two disadvantages of this approach are that, first, nondegradable, water-soluble polymers have been used, and this requires a tedious fractionation of the polymer to assure that the molecular weight of the polymer is below the renal excretion threshold, and, second, the drug must be chemically attached to the polymer, which in effect creates a new drug entity with consequent regulatory hurdles that must be overcome. The use of polymer conjugates in cancer diagnosis and treatment is discussed in Duncan et al., “The role of polymer conjugates in the diagnosis and treatment of cancer”, S. T. P. Pharma Sciences, 6(4), 237–263 (1996), and an example of an alginate-bioactive agent conjugate is given in U.S. Pat. No. 5,622,718.
An alternate approach has been described. In this approach, an AB or ABA block copolymer is prepared where the B-block is hydrophobic and the A-block is hydrophilic. When such a material is placed in water, it will self-assemble into micelles with a hydrophobic core and a hydrophilic shell surrounding the core. Such micelles have a diameter of about 100 nm, which is large enough that when they are injected intravenously, the micelles can not leave the normal vasculature, but they are small enough to leave the vasculature within tumors. Further, a 100 nm diameter is too small to be recognized by the reticuloendothelial system, thus enhancing micelle lifetime within the blood stream. Additionally, when the hydrophilic block is poly(ethylene glycol), further enhancement of circulation time is noted, as has been observed with “stealth” liposomes. The use of block copolymer micelles is reviewed in Kwon et al., “Block copolymer micelles as long-circulating drug delivery vehicles”, Adv. Drug Delivery Rep., 16, 295–309 (1995).
U.S. Pat. Nos. 5,412,072; 5,449,513; 5,510,103; and 5,693,751 describe block copolymers useful as micellar delivery systems where the hydrophilic block is polyethylene glycol and the hydrophobic blocks are various derivatives of poly(aspartic acid), poly(glutamic acid) and polylysine. U.S. Pat. Nos. 5,412,072 and 5,693,751 describe an approach where drugs have been chemically attached to the hydrophobic segment; while U.S. Pat. Nos. 5,449,513 and 5,510,103 describe an approach where hydrophobic drugs have been physically entrapped within the hydrophobic portion of the micelle. This latter approach is clearly preferable because no chemical modification of the drug is necessary.
Bioerodible Block Copolymer Matrix for Controlled Drug Delivery
In AB, ABA, or BAB block copolymers comprising a hydrophilic A block and a hydrophobic B block, the A and B blocks are incompatible and on a microscopic scale will phase-separate. This phase separation imparts unique and useful thermal properties to the material.
There is considerable prior art in the development of block copolymers comprised of poly(ethylene glycol) and bioerodible hydrophobic segments such as poly(L-lactic acid), poly(L-lactic-co-glycolic acid) copolymers and poly(ε-caprolactone), and discussion of their use as drug delivery agents. For example, see Wolthuis et al., “Synthesis and characterization of poly(ethylene glycol) poly-L-lactide block copolymers”, Third Eur. Symp. Controlled Drug Delivery 271–276 (1994), Youxin et al., “Synthesis and properties of biodegradable ABA triblock copolymers . . . ”, J. Controlled Release, 27, 247–257 (1993), and U.S. Pat. No. 5,133,739.
Poly(ortho esters) are known as potential vehicles for sustained release drug delivery. See, for example, Heller, “Poly(Ortho Esters)”, Adv. Polymer Sci., 107, 41–92 (1993), and references cited therein, and U.S. Pat. Nos. 4,304,767, 4,946,931, 4,957,998, and 5,968,543.
U.S. Pat. No. 5,939,453 describes block copolymers prepared from polyethylene glycols and certain poly(ortho esters).
The documents listed in this section and elsewhere throughout this application are incorporated into this application by reference.